Joint replacement surgery seeks to replace portions of a joint with prosthetic components so as to provide long-lasting function and pain-free mobility.
For example, in the case of a prosthetic total hip joint, the head of the femur is replaced with a prosthetic femoral stem component, and the socket of the acetebulum is replaced by a prosthetic acetabular cup component, whereby to provide a prosthetic total hip joint.
In the case of a prosthetic total knee joint, the top of the tibia is replaced by a prosthetic tibial component, and the bottom of the femur is replaced by a prosthetic femoral component, whereby to provide a prosthetic total knee joint.
Still other prosthetic total joints, and their constituent components, are well known in the art.
The present invention is directed to orthopedic prostheses for restoring the hip joint and, more particularly, to improved prosthetic femoral stem components.
Prosthetic femoral stem components typically comprise a proximal section for seating in the proximal section of the resected femur and presenting a ball for seating in the acetabular socket, and a distal section for seating in the femur's medullary canal so as to extend along the shaft of the femur.
It is, of course, important that the prosthetic femoral stem component make a proper fit with the surrounding bone. To this end, prosthetic femoral stem components are typically offered in a range of different sizes in an effort to accommodate variations in patient anatomy. However, despite this, it has been found that it can be difficult to provide the correct prosthetic femoral stem component for patients. This is due to the wide variation in patient anatomies and the practical limitations of hospital inventory. By way of example, where a femoral component is selected having a proximal section appropriately sized for the proximal section of the resected femur, the distal section of the prosthesis may not be appropriately sized for proper seating in the distal section of the femur. This can present serious problems for the patient, including problems relating to joint stability and pain.
On account of the foregoing, there has been substantial interest in forming prosthetic femoral stem components out of a plurality of separate elements, wherein each of the elements may be independently selected so as to most closely approximate patient anatomy, and wherein the separate elements may be assembled to one another in the operating room, using modular connections, so as to provide the best possible prosthetic femoral stem component for the patient.
Modular prosthetic femoral stem components are offered in a variety of configurations.
By way of example, a so-called “two part” modular prosthetic femoral stem component may comprise a body element and a combined neck-and-stem element. More specifically, the body element includes a central aperture through which the combined neck-and-stem element extends. The body element is selected so that its outer surface is appropriately sized for proper seating in the proximal section of a resected femur. The combined neck-and-stem element is selected so that when it is mounted to the body element and deployed within the femur, a ball located at the proximal end of the combined neck-and-stem element will be properly seated in the hip joint's corresponding acetabular cup while the distal end of the combined neck-and-stem element will be properly seated within the medullary canal of the femur. The body element and the combined neck-and-stem element are adapted to be secured to one another in the operating room, using modular connections, so as to form the complete modular prosthetic femoral stem component. Such modular connections are well known in the art.
Other types of “two part” modular prosthetic femoral stem components are also well known in the art.
By way of further example, a so-called “three part” modular prosthetic femoral stem component may comprise a body element, a neck element and a stem element. More specifically, the body element includes a central aperture into which portions of the neck element and the stem element extend. The body element is selected so that its outer surface is appropriately sized for proper seating in the proximal section of a resected femur. The neck element is selected so that when it is mounted to the remainder of the modular prosthetic femoral stem component and deployed within the femur, a ball located at the proximal end of the neck element will be properly seated in the hip joint's corresponding acetabular cup. The stem element is selected so that its outer surface is appropriately sized for proper seating within the medullary canal of the femur. The body element, the neck element and the stem element are adapted to be secured to one another in the operating room, using modular connections, so as to form the complete modular prosthetic femoral stem component. Again, such modular connections are well known in the art.
Other types of “three part” modular prosthetic femoral stem components are also well known in the art.
Thus it will be seen that the modular prosthetic femoral stem component generally comprises: (1) a motion structure that reproduces the motion of the original, natural joint; and (2) a load structure that transmits the loads caused by that motion (e.g., walking) to the remaining bone of the resected femur. The motion structure generally comprises the neck (and ball) portion of the modular prosthetic femoral stem component. The load structure generally comprises two portions: a body portion for the transmission of axial and torsional loads to the remaining bone of the resected femur, and a stem portion to assist the body portion in resisting bending loads placed upon the body portion. In this context, the body portion is the aforementioned body element of the aforementioned “two part” modular prosthetic femoral stem component and the aforementioned body element of the aforementioned “three part” modular prosthetic femoral stem component.
The goal of the body element of a modular prosthetic femoral stem component is to transmit loads to the remaining bone of the resected femur in the same regions that the bone originally carried those loads. At the ends of the bone, this is indicated by the areas of greatest cortical bone thickness or cancellous bone density. Bone grows in response to mechanical stress. Where bone is needed to resist load, it forms; where it is not needed to resist load, it is resorbed. This principle is known as Wolff's Law. The loads generated by joint motion are preferentially carried by the bone at the end nearest the joint. If a prosthesis bypasses this region in favor of loading the more central section of the bone, resorption at the end of the bone will result. This can eventually lead to fracture of the bone or loosening of the prosthesis.
Thus, in order to transfer loads to the correct region of the bone, and to transfer the loads uniformly in that region, the body element of the prosthesis should, ideally, closely but not exactly approximate the inner contour of the hard cortical bone. More particularly, it has been found that it is preferable to leave a small amount of compacted cancellous bone between the body element of the prosthesis and the cortical bone. This is because the cancellous bone is significantly more metabolically active than the cortical bone and, as such, able to more quickly establish bone ingrowth with the prosthesis and remodel so as to carry the load. The body element should also fill the inner canal of the bone to a large extent so as to help resist torsional loads placed upon it.